1. Field of Invention
The present invention relates to an aluminum-alloy rolled sheet having improved corrosion-resistance and weldability, and to a production method therefor. Particularly, the present invention relates to an aluminum-alloy rolled sheet which is suitable for using in such applications as an automobile-body sheet, where strength is particularly required and paint is applied to the sheet and then baked, and also relates to a production method for the same.
2. Description of Related Arts
The most frequently used sheets for forming the body and the like of an automobile have heretofore been cold-rolled steel sheet. In recent years, however, in order to decrease the weight of an auotomobile, there has been considerations for using aluminum-alloy rolled sheets for the body sheets. The requirements of body sheets for an automobile are: excellent formability in press-forming, particularly excellent elongation and bulging-formability, as well as free from Luder's marks during forming; and, high strength, particularly high strength after baking, to which the body sheet is necessarily subjected.
Various aluminum-alloy rolled sheets have been used for the shaped articles where strength is required. The principal kinds of such aluminum-alloy rolled sheets are classified as follows depending upon the alloy-components.
(A) Non-heat treatable type Al-Mg series alloys. They are 5052 alloy with 0 temper (2.2-2.8% of Mg, 0.15 to 0.35% of Cr, and the balance of Al and the unavoidable impurities) and 5182 alloy with 0 temper (0.20-0.50% of Mn, 1.0 to 5.0% of Mg, and balance of Al and unavoidable impurities).
(B) Heat treatable type series Al-Cu alloy. This is 2036 alloy with T.sub.4 temper (2.2-3.0% of Cu, 0.1-0.4% of Mn, 0.3-0.6% of Mg, and the balance of Al and unavoidable impurities).
(C) Heat treatable type Al-Mg-Zn-Cu series alloys with T.sub.4 temper. They are proposed in Japanese Unexamined Patent Publication Nos. 52-141409, 53-103914, and 57-98648. The Al-4.5 Mg-0.38Cu-1.46Zn-0.18 Fe-0.09Si series alloy explained in Nikkei New Material, 1986, 4-7, No. 8, pages 63-72, particularly page 64 belongs to this type.
(D) Heat treatable type Al-Mg-Si series alloys. They are 6009 alloy with T.sub.4 temper (0.4-0.8% of Mg, 0.6-1.0% Si, 0.15-0.6% of Cu, 0.2-0.8% of Mn, and balance of Al and unavoidable impurities), and 6010 alloy with T.sub.4 temper (0.6-1.0% of Mg, 0.8-1.2% Si, 0.15-0.6% of Cu, 0.2-0.8% of Mn, and balance of Al and unavoidable impurities). These alloys are proposed in Japanese Examined Publication No. 59-39499. This publication also proposes the material with T.sub.4 or T.sub.6 temper, which contains, one or more of Mn in an amount of 0.2-0.8%, Cr in an amount of 0.1-0.3%, and Zr in an amount of 0.05-0.15%, in addition to 0.4-1.2% of Si, 0.05-0.35% of Fe, and 0.1-0.6% of Cu. Japanese Examined Patent Publication No. 61-15148 proposes an alloy which is referred to as AC120 alloy and contains Si and Mg, whose content is within the four points, i.e., (a) 1% Si and 0.6% Mg, (b) 1.8% Si and 0.6% Mg, (c) 1.8% Si and 0.2% Mg, and (d) 1.2% Si and 0.2% Mg, as well as 0.3% or less of one or more of Cr, Mn, Zr, and Ti.
It is however difficult to satisfy all of the properties required for the body sheets of an automobile body by the above described conventional alloys.
The alloys of (A) have drawbacks in poor strength, Luder's marks formed during the forming, and strength-reduction during the baking process. The alloy of (B) has drawbacks in insufficient formability and reduced strength during the baking process.
The alloys of (C) have insufficient formability, especially bendability, and reduced strength during paint baking process. The alloys of (D) have features in that the Luder's marks do not generate and further the strength equivalent to that of cold-rolled steel sheets can be attained by baking. However, it is known that the elongation, which is an index of the formability, is considerably low.
Researches and studies have been made to provide aluminum-alloy rolled sheets which satisfy all of the following requirements of body sheets of an automobile body: excellent formability, particularly, excellent elongation and bulging formability, and free from Luder's marks; high strength, particularly post-baking strength; excellent corrosion-resistance; and, excellent weldability.
In the Al-Si-Mg series alloys, to which the present invention belongs, there are known alloys satisfying the above requirements in a rather high extent. However, the requirements for the aluminum-alloy rolled sheets must be severe, since they are less advantageous than the cold-rolled steel sheets from a view point of cost. There has not yet been provided an aluminum-alloy rolled sheet which satisfies all of the requirements. Particularly, the low elongation of the conventional aluminum-alloy rolled sheet is disadvantageous from a view point of formability.
Regarding the corrosion-resistance of aluminum-alloy used for a body sheet of an automobile, there is heretofore an opinion that, since the corrosion-resistance of aluminum-alloy is superior to that of steel, no problem in the corrosion-resistance would arise provided that the paint coating has no defects (Nikkei New Material, ibid.). In addition, the corrosion-resistance failure due to a defect of chromium plating applied on an aluminum sheet, i.e., the swelling failure of the chromium-plating layer, is tested by CASS method (Japanese Examined Patent Publication No. 59-39499 mentioned above). Although this opinion contemplates the corrosion-resistance required for the aluminum-alloy rolled sheet for forming, the corrosion-resistance required is made clearer than heretofore. Namely, it is made clear that the corrosion-resistance of unpainted sheet and pitting-corrosion resistance are required for the aluminum alloy, and, further, the peeling of paint coating (blister) and filiform corrosion should not occur during long service period. An aluminum-alloy rolled sheet, which has excellent corrosion-resistance and, in which excellent strength and formability are combined, are unknown.
The body sheets of an automobile are welded by spot welding in most cases but are welded by so-called arc welding by TIG or MIG depending upon the parts thereof. Since the thickness of an arc-welded sheet is usually 2.0 mm or less and hence relatively thin, welding is generally speaking difficult. Good weldability by arc welding is therefore required for an aluminum-alloy rolled sheet.
The present invention is made under the background described in the foregoing.